
I call that minor evap. I replace 2 gallons a day in my 60g system. I used to do 25 - 35g a day at work in one system and now we do it by the hundreds at my current job
I's invest in a auto top off unit, like Matt suggested. It'll really help solve this problem for you. I went a more expensive route myself and bought a scientific grade paristaltic dosing pump and just dialled it into my need.